diff --git a/include/msgpack/v1/adaptor/boost/fusion.hpp b/include/msgpack/v1/adaptor/boost/fusion.hpp
index 69a11240..73e1766b 100644
--- a/include/msgpack/v1/adaptor/boost/fusion.hpp
+++ b/include/msgpack/v1/adaptor/boost/fusion.hpp
@@ -111,7 +111,7 @@ struct as<
     template<std::size_t... Is, typename U>
     static std::tuple<
         typename std::remove_reference<
-            typename boost::fusion::result_of::at_c<T, Is>::type
+            typename boost::fusion::result_of::at_c<T, static_cast<int>(Is)>::type
         >::type...>
     to_tuple(U const& u, seq<Is...>) {
         return std::make_tuple(boost::fusion::at_c<Is>(u)...);